Introduction: Thirty years of ecosystem research have demonstrated unequivocally that certain species of plants, microbes, or animals can have dramatic effects on such ecosystem processes and properties as primary productivity, soil chemistry and structure, evapotranspiration, leaf area and fluxes of trace gases. However, the significance of having a particular number of species participating in a particular ecosystem process or continuum o ecosystem processes is less obvious. Apparently well-functioning natural ecosystems can be found in which most of the biomass is composed of one or two species of plants or other functional groups. Likewise, many ecosystems apparently maintain full functionality despite major changes in the identities and total number of species over time. Thus it is difficult to conclude that biodiversity per se has any particular significance for ecosystem processes. Many species are not necessarily better than few species for carrying out or maintaining a particular ecosystem process. Extensive evidence demonstrates that environmental conditions regulate biodiversity. Biodiversity varies predictably along elevational, disturbance, productivity, moisture, and latitudinal gradients (Huston, 1994). Although the diversity of many different taxa. reaches a maximum in lowland tropical rain forests, the reasons for these diversity maxima vary significantly among taxa. In fact, the response to any environmental factors known to influence diversity is likely to vary among taxa, with some taxa increasing in diversity and others decreasing in diversity in response to a single gradient or change in conditions. Biodiversity patterns can best be understood if they are subdivided into groups of species that are reasonably consistent in their interactions with each other, with other species, and with the physical environment. Such groups, which have been described variously as guilds, strategies, growth forms, and functional types, have long been an important concept in ecology. A major rationale for this subdivision of organisms is that interactions such as competition are more likely to be stronger among organisms of the same guild or functional type than among organisms of different functional types